To use it, highlight a word as the X selection, and click on the Webster button; if all goes well, you will soon see a popup containing the definition of the word.
By default, this client produces a button on your tkgoodstuff panel. But you can set preferences so that it will not, in which case you can use the (Tcl) command "WebsterDefine" as a Menu client item.
You can customize the URL if you have access to a better dictionary server. If you know of a web page that has a "form" to fill out to get a definition, to determine the needed URL you may have to look at the html source of the page. Typically, you will want something like:
http://www2.hti.umich.edu/bin/oed/oed-idx.pl?q1=@word@That's what I use, but I think only University of Michigan people can access that server. The Webster default is Merriam-Webster's server:
http://www.m-w.com/cgi-bin/mweb?book=Dictionary&va=@word@